CS3502, Data and Computer Networks: the physical layer-1

CS3502,Data and Computer Networks:the physical layer-1

physical layer - purpose • To transmit bits, by encoding them onto signals; and to receive the signals, interpreting them as bits input: sequence of bit S, from DL or MAC layer output: sequence of bit S’, to DL or MAC layer ideally, S = S’

Physical layer definitions • signal 1. a mechanism used to carry information over time OR distance 2. a sign or gesture giving information 3. a sequence of electrical impulses or waves 4. electromagnetic encoding of data

Signals • examples: • physical gesture, wave, hand signal • flashes of light (eg, Morse code) • sound: vary tone, loudness or duration • flags • smoke • mirrors • electrical voltages

transmission definitions 1. the action of conveying electrical or optical signals from 1 point to 1 or more other points in space 2. the process of sending information from 1 point to another • functions necessary for transmission systems • path for signal transfer (medium) • transform signal to appropriate form (code) • launch the signal (transmit) • remove, receive or detect the signal (receive)

transmission properties • functions present in some transmission systems (often helpful) • compensate for loss in media • compensate for distortion introduced by media • control access to the media (ie, protect from interference) • monitor and control quality of transmission

signal - can be modeled asf(t) t --> f(t) ... f(t) represents some physical quantity: voltage, amplitude, frequency, etc.

signals • functions/signals may be either --- • continous • no breaks in the graph or • discrete • takes only a finite or countable number of values Q: can a function be both? Q: must a function be one or the other?

examples of functions f(t) = 1 f(t) = c f(t) = Roof(t) f(t) = Floor(t) f(t) = t - Floor(t) f(t) = sin(t) • you should be able to graph all of these

digital/ analog signals • digital signal 1. assumed to take on finite number of values, AND 2. has meaning only at discrete points in time. • digital signals represented by discrete functions. (however this is an idealized and somewhat unrealistic picture).

digital/analog signals • analogy: a partial likeness between 2 things that are compared (Oxford Dict.) • analog signal: 1. a signal that is an analog of the quantity being represented; eg, signal voltage proportional to volume of sound 2. continuous range of values 3. continuous write time; always valued.

digital/analog signals • digital data: text, bits; discrete valued. • analog data: sound, vision; music, etc. continuous valued. Note: digital (analog) signals can transport both digital and analog data, so 4 combinations (DD,DA,AD,AA) possible • some media only propagate analog signals efficiently, and sometimes more efficient to digitize analog data

digital/ analog signals • periodic function -- cyclical in values (note mathematical definition) • 3 key properties of periodic signals: • amplitude: instantaneous value • frequency: cycles per second (hertz) • phase: position within a cycle/period • these quantities are varied, in order to use the signal to carry information

digital/ analog signals • key fact: any signal can be represented as a sum (possibly infinite) of periodic functions. (Fourier analysis mathematical picture) • f(t) =(1/2)·k0 +n=1.infan·sin(2··n·f·t) + n=1.infbn·cos(2··n·f·t) • an = 2/Tt=0…Tf(t) ·sin(2··n·f·t) ·dt • bn = 2/Tt=0…Tf(t) ·cos(2··n·f·t) ·dt • k0 = 2/Tt=0…Tf(t) ·dt ; the average amplitude

Tuned to 7·f Tuned to 6·f Tuned to 5·f Tuned to 4·f Tuned to 3·f Tuned to 2·f Tuned to 1·f digital/ analog signals .5·sin(2··7·f·t) • (Fourier analysis graphical picture) 1·sin(2··6·f·t) 5·sin(2··5·f·t) 1·sin(2··4·f·t) 2·sin(2··3·f·t) f(t) 4·sin(2··2·f·t) 8·sin(2··1·f·t) a1=8

transmission media • transmission medium: the physical element through which signals must pass, from transmitter to receiver • examples: air, water, (outer) space, copper wires, optical fiber • two main categories: guided and unguided • propagation delays of signals in media

transmission terminology • direct link • simplex • half-duplex • full duplex • spectrum - range of frequencies making up a signal • bandwidth - width of the spectrum; range of frequencies • examples

transmission terminology • note 1: bandwidth key factor in determining data rate; • note 2: however do not confuse bandwidth (hertz) and and data rate (bps) • attenuation • amplifier • boosts energy of analog signal; increases amplitude • makes no distinction between noise and signal • repeater • receives, interprets and repeats a digital signal • adds little or no noise/distortion

transmission terminology • repeater-amplifier diagram comparison

modems, codecs, bauds, bits • modem (modulator-demodulator) • translates a digital signal (bit) into an analog signal, for transmission as an analog signal; receives the corresponding analog signal, and translates back into digital (bit) • purpose: use analog medium for digital data/signals • example: PC modem, phone lines; TV cable modems • techniques: PSK, ASK, FSK and combinations.

modems, codecs, bauds, bits • codec (codec/decoder) • converts analog data into digital form (bits), and the reverse. • two main techniques: PCM, DM • PCM (pulse code modulation) • absolute values, based on sampling theorem; (nearly) total information • DM (delta modulation) • based on differences; less accurate

modems, codecs, bauds, bits • Baud rate -maximum number of times per second that the signal element can change • Baud - The unit in which the Baud rate is measured • incorrect to say “9800 bauds per second.” • thus, the baud rate is the rate at which distinct signal elements are sent. • also called “signaling rate” • applies to digital signals or analog signals carrying digital data.

modems, codecs, bauds, bits • diagram: bauds and bits Bit rate = baud rate·log2(#of signal levels) 3 2 1 0 Signal levels Example: A 9600 baud modem has a baud rate of 9600 baud. If it uses two signal levels is also runs at 9600 bps. T = 1/Baud Rate measured in baud

CS3502, Data and Computer Networks: the physical layer-1